1. Field of the Invention
The present invention relates to a light wavelength converter, which is used in an information processor, such as an optical memory disc system or a laser beam printer, and an optical application measuring apparatus using laser beams emitted from a semiconductor laser device when the wavelength of the laser beams is converted into a short wavelength zone.
2. Description of the Prior Art
For an information processor, such as an optical memory disc system or a laser beam printer, and an optical application measuring apparatus, the laser beam of a semiconductor laser device that is easily condensed and superior in directivity is used. In general, the oscillation wavelength of semiconductor laser devices is 780 nm or 830 nm, the laser beam being near infrared. In recent years, in order to increase the amount of information to be processed in an information processor, or to improve the measurement accuracy of the optical application measuring apparatus, the laser beam has been promoted to be of a short wavelength. For example, in an information processor, such as an optical memory disc system or a laser beam printer, the laser beam emitted from the semiconductor laser device is condensed at a predetermined place so as to write the information or images. The wavelength of the laser beam and the diameter of the focusing spot usually have therebetween a proportional relationship, so that, when the wavelength of a laser beam becomes short, the diameter of the focusing spot can be reduced. When the diameter thereof is reduced, the amount of information (i.e., the recording density) to be written into the optical memory disc system can be increased. Moreover, the laser beam printer can form micro-images when the wavelength of the laser beam is shortened, so that the recording density can be increased and the resolution can be improved. Furthermore, if green or blue laser beams with shorter wavelengths could be easily obtained, they could be combined with the red laser beams currently in use to realize high-speed, high-resolution color printers. In optical application measuring apparatuses, the measuring precision can be improved by shortening the wavelength of the laser beam.
In recent years, laser beams with oscillation wavelengths in the 600-nm range (680 nm) have been obtained with semiconductor laser devices using group III--V semiconductor materials, but further shortening of the wavelength with group III--V semiconductor materials is difficult. Semiconductor laser devices using ZnSe, ZnS and other group II--VI semiconductor materials are being studied, but p-n junctions have not yet been realized. Therefore, semiconductor laser devices capable of oscillating shortwave green and blue laser beams have not yet been realized because suitable materials have not yet been discovered. For this reason, green, blue and other shortwave laser beams are currently obtained only with argon ion laser and other large gas lasers.
In order to obtain green and blue shortwave laser beams without using large gas lasers, light wavelength converters have been proposed which yield laser beams with half the wavelength of laser beams oscillated by solid-state lasers and semiconductor laser devices. The light wavelength converters utilize non-linear optical phenomena typified by second harmonic generating (SHG) using crystals with a non-linear optical effect, whereby a laser beam is output with a wavelength one half that of the input fundamental wave.
A conventional light wavelength converter has, as shown in FIG. 3, a channel optical waveguide 32 formed on a LiNbO.sub.3 crystalline substrate 31 by a proton-exchange technique. The LiNbO.sub.3 crystalline substrate 31 has a large non-linear optical constant, so a second harmonic wave 34 of the half-wavelength of the wavelength of the fundamental wave 33 incident on the optical waveguide 32 is generated by the SHG phenomenon. This second harmonic wave 34 emanates from the optical waveguide 32 into the LiNbO.sub.3 crystalline substrate 31 at an angle .theta. which is phase-matched with the fundamental wave 33, and it is then emitted from the end of the substrate 31.
By means of this converter, the generation of a green 0.53-.mu.m laser beam has been observed using a YAG laser with an oscillation wavelength of 1.06 .mu.m. Moreover, by optically coupling a laser beam emitted from a semiconductor laser device, which has an oscillation wavelength of 0.84 .mu.m and an optical output of 80 mW, with one end of the optical waveguide at 40 mW by means of an optical system including lenses and prisms, a blue laser beam with a wavelength of 0.42 .mu.m and an optical output of 0.4 mW has been observed. In the case of a laser beam with a fundamental wave of multiple oscillation wavelengths (oscillation in a longitudinal mode), a sum wave (sum of optical frequencies of two fundamental waves) is generated. Below, second harmonic waves also refer to sum waves.
It is known that the output of a second harmonic wave generated in this way is proportional to the square of the output of the input fundamental wave. Therefore, in order to obtain a blue laser beam with a practical output of 5 mW using the above-mentioned light wavelength converter, it is necessary to input a fundamental wave with an output of approximately 140 mW into the optical waveguide 32 and propagate it there. Considering the coupling efficiency of the fundamental wave with the optical waveguide, a semiconductor laser device with an output of approximately 280 mW is required. However, the output of high-output semiconductor laser devices is only around 50.about.100 mW at the present time. Moreover, due to the low coupling efficiency thereof with the optical waveguide, it is difficult to obtain a blue laser beam with a practical output of 5 mW.